Method for manufacturing semiconductor device

ABSTRACT

A method for manufacturing a semiconductor device includes at least forming a lower electrode comprising titanium nitride on a semiconductor substrate, forming a dielectric film comprising zirconium oxide as a primary constituent on the lower electrode, forming a first protective film comprising a titanium compound on the dielectric film, and forming an upper electrode comprising titanium nitride on the first protective film. The method can include a step of forming a second protective film on the lower electrode before the step of forming the dielectric film on the lower electrode.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to a semiconductor device and a method ofmanufacturing the same, and, in particular, to dynamic random accessmemory (DRAM) having a capacitor with properties of low leakage currentand high permittivity.

2. Description of Related Arts

DRAM has been used for a semiconductor memory operable at a high speedin a computer or other electronic devices. DRAM has a memory cell arrayand a peripheral circuit for operating the array. The memory cell arrayhas a plurality of units arranged in a matrix, and each unit comprisesone switching transistor and one capacitor.

As in other semiconductor devices, DRAM has developed withminiaturization of each cell to satisfy a demand for high-integration.As a result, the area on which a capacitor is formed decreases, and itis thus difficult to ensure the capacity required for a memory device.To solve this problem, a three-dimensional structure of electrodes,upper and lower electrodes made of a metallic material (MIM structure),a capacitive insulation film having high permittivity, etc have beenintroduced. Currently, DRAM with a minimum feature size (F value) of 70nm or less, which is used as a standard index of a technology level,necessarily has a three-dimensional electrode structure, and upper andlower electrodes made of a metallic material have already been used inpractice. Therefore, the prospect of improving the features of acapacitor on the basis of these technical developments is bleak. Thecurrent trend of additional miniaturization mainly includes improvingthe feature of a capacitor by high permittivity of a capacitiveinsulation film left for the last.

Recently, a capacitor with a MIM structure, e.g., a structure ofTiN/ZrO₂/TiN, has been used as a DRAM capacitor.

DRAM is formed from heat treatment at 450° C. to 500° C. as anunavoidable process after a capacitor has been formed. However, adielectric film made of a single body of zirconium oxide cannot achievea sufficient thermostability, and leakage current likely increases afterheat treatment.

Therefore, several attempts have been made to increase thermostability,and the examples of such attempts include a multilayer dielectric film,e.g., a ZAZ structure where Z and A mean ZrO₂ and Al₂O₃, respectively,in ZrO₂/Al₂O₃/ZrO₂, or a laminating layer having a plurality of Al₂O₃and ZrO₂ layers alternately.

This structure aims to accomplish the desirable characteristic from thecombination of zirconium oxide (ZrO₂) having high permittivity andaluminum oxide (Al₂O₃) having high thermostability instead of highpermittivity.

For example, JP 2006-135339 A discloses an AZ, ZA, or ZAZ structure, ora method for forming a multilayer dielectric film alternately laminatinga ZrO₂ thin film and an Al₂O₃ thin film.

In practical DRAM, a heat treatment with a process of forming an upperelectrode, which is to be performed after forming a dielectric film of acapacitor, or a heat treatment such as hydrogen annealing for reducingthe interface state of a transistor, is carried out. However, this heattreatment, which is performed after forming a dielectric film comprisingzirconium oxide such as a ZAZ structure as a principal component, maycause leakage current of a capacitor. This result may imposerestrictions on the manufacturing process, and make it difficult toachieve a good characteristic of transistors by a sufficient hydrogenannealing. As described herein, a dielectric film having zirconium oxideas a primary constituent means a dielectric film having a compositeratio of 0.8 or more, the composite ratio being indicated Z/(Z+M) whereZ is the number of zirconium atoms and M is the number of metallic atomsother than zirconium in the dielectric substance.

The inventors in this invention found out that the increase in leakagecurrent in a capacitor, which has been caused with the heat treatment,may occur due to the combination of (1) the degradation of thedielectric film itself by the increase of oxygen deficit in a dielectricfilm or by the diffusion of impurities such as nitrogen, (2) the partialdesquamation of a dielectric film, which has zirconium oxide as aprimary constituent, from titanium nitride electrodes, and (3) damages,such as crack, on a dielectric film itself. These events are consideredto stem from a change in stress caused particularly by the heattreatment under reducing atmosphere or by the secondary growth ofcrystal grains of a dielectric film.

SUMMARY

The inventors found that a capacitor having a dielectric film, whoseprimary constituent is zirconium oxide of, for example, a ZAZ structuremay prevent the increase in leakage current in spite of a heat treatmentduring the formation of an upper electrode and a hydrogen annealing fordecreasing the interface state of transistors, if the capacitor isformed by forming a dielectric film comprising zirconium oxide in amicrocrystal state as a primary constituent, forming a first protectivefilm having titanium oxide as a primary constituent under a conditiondevoid of the secondary growth of crystal grains, and then forming anupper electrode. As described herein, the secondary growth of crystalgrains means the rearrangement of constituent atoms by, for example, aheat treatment after film formation, and the change into larger crystalgrains by the reformation of grain boundary.

Additionally, a sufficient tolerance against an inevitable heattreatment in a DRAM manufacturing process has been found to beaccomplished by further providing a second protective film between alower electrode and a dielectric film comprising zirconium oxide as aprimary constituent, the second protective film comprising titaniumoxide as a primary constituent.

Specifically, one embodiment of the invention provides a method formanufacturing a semiconductor device including a formation of acapacitor, wherein the formation of the capacitor comprises at least:

forming a lower electrode comprising titanium nitride on a semiconductorsubstrate,

forming a dielectric film comprising zirconium oxide as a primaryconstituent on said lower electrode,

forming a first protective film comprising a titanium compound as aprimary constituent on said dielectric film, and

forming an upper electrode comprising titanium nitride on said firstprotective film.

Further, another embodiment of the invention provides a method formanufacturing a semiconductor device including a formation of acapacitor, wherein the formation of the capacitor comprises at least:

-   -   forming a lower electrode comprising titanium nitride on a        semiconductor substrate,    -   forming a second protective film on said lower electrode,    -   forming a dielectric film comprising zirconium oxide as a        primary constituent on said second protective film,    -   forming a first protective film comprising a titanium compound        on said dielectric film, and    -   forming an upper electrode comprising a titanium nitride on said        first protective film.

According to the invention, the structure of a protective film insertedinto the interface between the titanium nitride electrode and thedielectric film comprising zirconium oxide as a primary constituent canprohibit damages to the dielectric film caused by heat treatment duringthe formation of the upper electrode or by annealing for decreasing theinterface state of transistors. As a result, restrictions, such as anupper limit in process temperature, imposed on a manufacturing processcan be alleviated, and the characteristic of transistors can thus becompatible with that of capacitors. Accordingly, high reliability and ahigh yield rate can be accomplished for a device.

BRIEF DESCRIPTION OF DRAWINGS

The above features and advantages of the present invention will be moreapparent from the following description of certain preferred embodimentstaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic, cross-sectional view of a capacitor structure inthe related art, showing a structure of a single layer ZrO film.

FIG. 2 is a graph illustrating the characteristic of leakage current inthe capacitor of FIG. 1.

FIG. 3 is a schematic, cross-sectional view of a capacitor structure inthe related art, showing a ZAZ structure.

FIG. 4 is a graph illustrating the characteristic of leakage current inthe capacitor of FIG. 3.

FIG. 5 is a schematic cross-sectional view of a flat capacitor whichapplies to a capacitor structure having a single layer ZrO film in orderto evaluate the effect of a protective film used to a capacitorstructure according to one embodiment of the invention.

FIG. 6 is a graph illustrating the characteristic of leakage current inthe capacitor of FIG. 5 to which a titanium oxide film applies as aprotective film.

FIG. 7 is a graph illustrating the characteristic of leakage current inthe capacitor of FIG. 5 to which a titanium nitride film applies as aprotective film.

FIG. 8 is a schematic cross-sectional view of a flat capacitor in orderto evaluate a capacitor structure according to one embodiment of theinvention.

FIG. 9 is a graph illustrating the characteristic of leakage current inthe capacitor of FIG. 8 to which a titanium oxide film applies as aprotective film.

FIG. 10 is a schematic cross-sectional view of a flat capacitor in orderto evaluate a capacitor structure according to another embodiment of theinvention.

FIG. 11 is a graph illustrating the characteristic of leakage current inthe capacitor of FIG. 10 to which a TiO film applies as a protectivefilm.

FIG. 12 is a view illustrating the relationship between the leakagecurrent characteristic and an EOT.

FIG. 13 is a cross-sectional schematic view generally illustrating theentire structure of DRAM, which is semiconductor memory device in theinvention.

FIG. 14 is a top, cross-sectional view of the structure of FIG. 13 takenalong line X-X.

FIGS. 15A to 15I are cross-sectional views illustrating a manufacturingprocess for the capacitor of FIG. 13.

FIG. 16 is a view illustrating a XRD result before annealing in a samplestructure of FIGS. 18A and 18B.

FIG. 17 is a view illustrating a XRD result after annealing in a samplestructure of FIGS. 18A and 18B.

FIGS. 18A and 18B are views illustrating sample structures used toobtain the data of FIGS. 16 and 17.

DETAILED DESCRIPTION OF REFERRED EMBODIMENTS

The invention will be now described herein with reference toillustrative embodiments. Those skilled in the art will recognize thatmany alternative embodiments can be accomplished using the teachings ofthe present invention and that the invention is not limited to theembodiments illustrated for explanatory purpose.

EXPERIMENT 1

First, a capacitor having a single layer of a ZrO₂ film (hereinafterreferred to as “ZrO film”) has been evaluated.

FIG. 1 shows the structure of a flat capacitor including lower electrode102 made of a titanium nitride film (TiN film), upper electrode 104 madeof a TiN film in the same way, and dielectric film 103 made of a ZrOfilm sandwiched between the upper and lower electrodes.

Lower electrode 102 made of a TiN film has been formed using a chemicalvapor deposition (CVD) method with reaction gases of titaniumtetrachloride (TiCl₄) and ammonia (NH₃) in consideration of theapplication thereof to a three-dimensional structure. The depositiontemperature was 450° C., and the thickness of the film was 10 nm.Hereinafter, a TiN film formed by a CVD method is referred to as aCVD-TiN film.

The ZrO film, which is to be dielectric film 103, has been formed usingan atomic layer deposition (ALD) method with a reaction gas of ozone(O₃) and a Zr precursor of TEMAZ, i.e., tetrakis(ethylmethylamino)zirconium: Zr[N(CH₃)CH₂CH₃]₄, which is an organometallic complex. Thetemperature of forming the film was 250° C. and the film thickness was 6nm. Dielectric film 103 is formed by repeating a fundamental sequenceuntil a desired film thickness is obtained, the sequence including thesteps of introducing the Zr precursor into a reaction chamber in which asemiconductor substrate is installed and adsorbing the Zr precursor onthe surface of the lower electrode as one atomic layer, nitrogen-purgingthe remaining precursor in a gas phase, introducing ozone and oxidizingthe adsorbed precursor, and nitrogen-purging the remaining ozone in agas phase.

Upper electrode 104 made of a TiN film has been formed using a masksputtering method with a known area. The mask sputtering method is toset a flat mask on the top surface of the ZrO film, to deposit a TiNfilm (hereinafter referred to as “PVD-TiN film”) thereon by a sputteringmethod, and to form an upper electrode in dot shape. The depositingtemperature was room temperature, and the film thickness was 10 nm.

The curve indicated as Reference B in FIG. 2 illustrates thecharacteristic of leakage current when a voltage between −3 V and +3 Vis applied to upper electrode 104 in the capacitor structure explainedabove. It is notified that +2.3 V and −2.2 V became the voltages appliedto meet the index of current density, i.e., 1 E-7 (A/cm²). The abovecapacitor shows the leakage current characteristic having a sufficientmargin in that the leakage current standard, which is allowable for useas a semiconductor device, is equal to 1 V or more in both positive andnegative values at the current density level.

The curve indicated as Reference A in FIG. 2 shows a result where theupper electrode uses a CVD-TiN film as does the lower electrode insteadof a PVD-TiN film. As clearly depicted in the figure, the leakagecurrent increases by seven exponents under the structure with the upperelectrode made of a CVD-TiN film compared to the structure with theupper electrode made of a PVD-TiN film. The capacitor in this case makesit difficult to store information therein, and cannot be in use.

For application to a three-dimensional capacitor, as stated above, theupper electrode, as well as the lower electrode, should be formed by aCVD method, which ensures good step coverage. However, thecharacteristic indicated by Reference A has a substantially large amountof leakage current, thereby failing to make a semiconductor device inuse.

The inventors here have examined the difference in the methods of theupper electrode, i.e., several conditions in sputtering and CVD methods,to find what condition influences the leakage current in the ZrO film,which forms a dielectric film, to change severely. As a result, theprimary causes of severely changing leakage current are assumed as thetemperature of forming a film and the environment therearound. That is,the sputtering method establishes the environment of inert gasesincluding argon (Ar) at room temperature in the sputter method, whilethe CVD method establishes the environment of titanium tetrachloride gas(TiCl₄) and ammonia gas (NH₃) and hydrogen chloride gas (HCl) orhydrogen gas (H₂) which are generated by reaction of titaniumtetrachloride gas (TiCl₄), ammonia gas (NH₃). These environments werethought to be the primary cause.

In other words, it was found that the leak characteristic of a capacitorhaving a dielectric film made of a crystallized zirconium oxide filmdepends largely on the method of forming the upper electrode.

The CVD-TiN film as the upper electrode is assumed to impose certaindamage on the ZrO film of the dielectric film in a film forming process.

When forming a film, a CVD-TiN is different from a PVD-TiN in that it isexposed to titanium tetrachloride (TiCl₄) and ammonia (NH₃), or gasesgenerated by reaction thereof, at a temperature of 380° C. to 650° C.Therefore, this difference in film forming conditions is thought toinfluence on the characteristic.

However, the PVD-TiN cannot be applied to a capacitor in athree-dimensional structure having a high aspect ratio, which iscurrently used for DRAM, because of inferior step coverage of thePVD-TiN.

EXPERIMENT 2

A capacitor with a ZAZ structure has been evaluated. The ZAZ structureis one of the dielectric film structures having zirconium oxide as aprimary constituent, and one of the dielectric film structure withinwhich aluminum oxide is included. A capacitor using a dielectric film ofa ZAZ structure is referred to as a capacitor with a ZAZ structure.

FIG. 3 illustrates a flat capacitor including lower electrode 102 madeof a CVD-TiN film, upper electrode 104 identically made of a CVD-TiNfilm, first dielectric film 105 made of a ZrO film formed on lowerelectrode 102, second dielectric film 106 made of an aluminum oxide film(hereinafter referred to as “AlO film”) formed on first dielectric film105, and third dielectric film 107 made of a ZrO film formed on seconddielectric film 106.

Lower electrode 102 made of a CVD-TiN film has been formed using a CVDmethod with reaction gases of titanium tetrachloride (TiCl₄) and ammonia(NH₃) in consideration of the application thereof to a three-dimensionalstructure. The deposition temperature was 450° C., and the thickness ofthe film was 10 nm.

The ZrO film, which is to be first dielectric film 105, has been formedusing an atomic layer deposition (ALD) method with a reaction gas ofozone (O₃) and a Zr precursor of TEMAZ, i.e., tetrakis(ethylmethylamino)zirconium: Zr[N(CH₃)CH₂CH₃]₄, which is a organometallic complex. Thetemperature of forming the film was 250° C. and the film thickness was 3nm. First dielectric film 105 is formed by repeating a fundamentalsequence until a desired film thickness is obtained, the sequencecomprising the steps of introducing the Zr precursor into a reactionchamber in which a semiconductor substrate is installed and adsorbingthe Zr precursor on the surface of the lower electrode as an atomiclayer, nitrogen-purging the remaining precursor in a gas phase,introducing ozone and oxidizing the adsorbed precursor, andnitrogen-purging the remaining ozone in a gas phase. The oxidizing agentmay be a mixture gas of ozone (O₃) and oxygen (O₂), or water (H₂O).

The AlO film, which is to be second dielectric film 106, has been formedusing an ALD method at 250° C. with an Al precursor of TMA (trimethylaluminum). This sample performed five film forming processes (i.e.,approximately 0.5 nm) in an ALD cycle. Second dielectric film 106 isformed by repeating a fundamental sequence until a desired filmthickness is obtained, the sequence comprising the steps of introducingthe Al precursor into a reaction chamber in which a semiconductorsubstrate is installed and adsorbing the Al precursor on the surface offirst dielectric film 105 as an atomic layer, nitrogen-purging theremaining precursor in a gas phase, introducing ozone and oxidizing theadsorbed precursor, and nitrogen-purging the remaining ozone in a gasphase. The oxidizing agent may be a mixture gas of ozone (O₃) and oxygen(O₂), or water (H₂O).

Then, a ZrO film of third dielectric film 107 was formed as is firstdielectric film 105. The film thickness was 3 nm. First and thirddielectric films 105 and 107 had a film thickness of 3 nm, respectively,in this experiment, but they are not necessarily to have the same filmthickness. For example, first dielectric film 105 can have a filmthickness of 5.0 nm while third dielectric film 107 can have a filmthickness of 1.0 nm. As such, both films can have an asymmetricalstructure.

As in lower electrode 102, upper electrode 104 made of a CVD-TiN filmhas been formed by a CVD method at 450° C. and to have a film thicknessof 10 nm.

I-V characteristic of the capacitor formed as above was evaluated, andis illustrated in FIG. 4. In FIG. 4, Reference C shows the I-Vcharacteristic of the capacitor measured after the upper electrode hasbeen formed, Reference D shows the I-V characteristic of the capacitormeasured after the upper electrode has been formed and then additionallyannealed with nitrogen at 450° C. for six hours, and Reference E showsthe I-V characteristic of the capacitor measured after the upperelectrode has been formed, annealed with nitrogen at 450° C. for sixhours and annealed with hydrogen at 450° C. for two hours.

With respect to Reference C, the applied voltage is −1.8 V or +2.0 V tomeet the index current density of 1 E-7 (A/cm²). It is understood thatthe characteristic improves compared to Reference B as well as ReferenceA depicted in FIG. 2.

Reference D shows a better leakage current characteristic in that theapplied voltage is −2.8 V or +2.4 V to meet the index current density of1 E-7 (A/cm²). This characteristic is thought to result from a furtherdensification of a dielectric film by heat treatment. However, ReferenceE illustrating the status after hydrogen annealing for two hours showsthat the current density of 1 E-7 (A/cm²) cannot be reached, and that apractical capacitor cannot be obtained.

Based on the results of Experiments 1 and 2, the significantdeterioration in a leakage current characteristic after hydrogenannealing of a dielectric film having a ZAZ structure for two hours isthought to be caused by a probable damage by a CVD-TiN film on a ZrOfilm of the third dielectric film during a process of forming the upperelectrode, just as about a single ZrO film depicted in Experiment 1.

A ZrO film formed by an ALD method is in a microcrystal state shortlyafter it has been formed at 250° C. When this ZrO film in a microcrystalstate is heat treated at a higher temperature than the film formingtemperature, a secondary growth of crystal grains occurs. The secondarygrowth of crystal grains depends on the thickness of a film, and, giventhe same condition for heat treatment, a thicker film leads apolycrystalline structure having a larger grain size.

In the meantime, the microcrystal state means a state where a cleargrain boundary is not observed in an image from a transmission electronmicroscope while a small peak caused by a crystal is observed in X-raydiffraction (XRD).

The leakage current is assumed to deteriorate as depicted Reference A inFIG. 2 because the single layer of a ZrO film has relatively thick andchanges into a polycrystalline structure having a larger grain size. Incontrast, a ZAZ structure interposes an AlO film between ZrO films andthus constitutes relatively thin ZrO films without a polycrystallinestructure having a large grain size. Furthermore, because an AlO filmhas a relatively high crystallization temperature and thus remains in anamorphous phase without crystallization at a temperature of asemiconductor process, an interfacial debonding or a crack by stressrelaxation are assumed to be prevented. Accordingly, during annealing inan inert environment and after the formation of a CVD-TiN film as anupper electrode, the expansion of damage by the secondary growth incrystal grains of a ZrO film is thought to be prevented by the presenceof an amorphous AlO film. However, annealing for a long time in ahydrogen gas environment loses the effect by the AlO film of preventingthe expansion of damage and is thus thought to lead the deterioration ofleakage current.

Therefore, in order to prevent the damage caused by the secondary growthin particle grains of a ZrO film, it was assumed to be desirable tocover the surface of the ZrO film with a protective film at atemperature at which the secondary growth of particle grains of the ZrOfilm decreases, preferably at a temperature at which the secondarygrowth of particle grains of the ZrO film does not substantially occur,and then to form a CVD-TiN film as an upper electrode.

Here, it was found that a titanium compound was effective as aprotective film. In particular, a titanium oxide, which is an oxide, anda titanium nitride, which is also used as an upper electrode, arepromising.

EXPERIMENT 3

First, the inventors have been examined the effect of a titanium oxidefilm (hereinafter, a “TiO film”) as a protective film.

FIG. 5 shows a capacitor structure including, on semiconductor substrate101, which is mono-crystalline silicon, lower electrode 102 made of aCVD-TiN film, dielectric film 103 made of a polycrystalline ZrO film,first protective film 110 made of a TiO film, and upper electrode 111made of a CVD-TiN film. The capacitor structure in this experiment isnot three-dimensional semiconductor memory device as explained above,and is constructed as a flat capacitor to achieve an easilymanufacturable structure for evaluating its characteristics. For thepurpose of comparison with Experiment 1, the dielectric film does nothave a ZAZ structure, but a single film of a ZrO film.

As in Experiment 1, lower electrode 102 made of a CVD-TiN film is formedon semiconductor substrate 101 with a thickness of 10 nm, and a ZrO filmis then formed for dielectric film 103. As in Experiment 1, the ZrO filmis formed to have a thickness of 6 nm by an ALD method using TEMAZ andozone at 250° C. The ZrO film just formed by the ALD method is in amicrocrystalline state. TEMAZ was used for a Zr precursor here, but theprecursor is not limited to TEMAZ. Ozone was used as a reaction gas, butthe reaction gas is not limited to ozone. For example, the reaction gascan be H₂O (steam). Dielectric film 103 has a thickness of 6 nm here,but may have a different thickness, for example, a thickness between 5and 8 nm.

It is preferable to set a film forming temperature within the range from210° C. to 280° C. No reaction will take place below 210° C., and adecomposition reaction will occur in a gas phase if a temperature isabove 280° C. In both cases, a film is difficult to be formed.

While Experiment 1 illustrates a CVD-TiN film formed on a ZrO film,Experiment 3 here shows that a first protective film made of a TiO filmis formed at a temperature at which no crystal growth of a ZrO filmoccurs. The first protective film is formed to have a thickness of 1 nmby an ALD method at 250° C. with a reaction gas of ozone and a Tiprecursor of TTIP (titanium tetra-isopropoxide: Ti(OCHMe₂)₄).Specifically, the film forming process by an ALD method includes thesteps of (1) introducing the Ti precursor in a reaction chamber where asemiconductor substrate is installed and then adsorbing the Ti precursoron the surface of the microcrystalline ZrO film, which is to be adielectric film, as an atomic layer level, (2) nitrogen-purging theremaining Ti precursor in a gas phase, (3) introducing ozone andoxidizing the adsorbed Ti precursor, and (4) nitrogen-purging theremaining ozone in a gas phase. The film formation was developed byrepeating the fundamental sequence including the four steps above untilthe film has a thickness of 1 nm. A film formation by an ALD method isdesirable in that it has a better step coverage and is easily applicableto a three-dimensional structure because the formation uses a surfaceadsorption reaction. A TiO film at the stage of film formation by an ALDmethod is in an amorphous state. Here, TTIP was used as a Ti precursor,but the Ti precursor is not limited to TTIP. TiMCTA(methylcyclopentadienyl tris(dimethylamino) titanium: (MeCp)Ti(NMe₂)₃)can be used for the Ti precursor. Ozone was used for a reaction gas, butthe reaction gas is not limited to ozone, and may use, for example, H₂O.The film formation temperature was set 250° C., but may be preferablywithin the range from 210° C. to 280° C. No reaction will take placebelow 210° C., and a decomposition reaction will occur in a gas phase ifa temperature is above 280° C. In both cases, an ALD film formation isprohibited.

Then, a CVD-TiN film, which is to be upper electrode 111, was formed. Asin lower electrode 102, the CVD-TiN film for upper electrode 111 wasformed to have a thickness of 10 nm by a CVD method at 380° C. to 600°C., preferably at 450° C., in consideration of its application to athree-dimensional structure.

After upper electrode 111 was formed, a mask material (not shown) with aknown area has been formed on upper electrode 111, and upper electrode111 was etching-removed using the mask material as a mask. As a result,a capacitor structure was formed as shown in FIG. 5.

FIG. 6 illustrates the leakage current characteristic of the capacitorshown in FIG. 5. The horizontal axis indicates a voltage applied to theupper electrode, and the vertical axis shows a leakage current value perunit area, which corresponds to an applied voltage. The characteristicidentified by Reference A is a leakage current characteristic when a ZrOfilm as depicted as Reference A in FIG. 2 has a thickness of 6 nm.Reference F illustrates a leakage current characteristic when thecapacitor includes first protective film 110 made of a TiO film with athickness of 1 nm. When a voltage of +1 V is applied, the leakagecurrent without first protective film 110 (Reference A) is 2 E-2(A/cm²),and the leakage current with the first protective film 110 (Reference F)is 7 E-8 (A/cm²). As specified from the comparison in FIG. 6, acapacitor with first protective film 110 made of a TiO film having athickness of 1 nm decreases the leakage current by five exponents,compared to one without first protective film 110, and shows dramaticimprovement.

The result represents that, during a process for forming the upperelectrode at a temperature of 450° C., the TiO film functions as aprotective film for effectively preventing the generation of damage inheat treatment under a reducing environment or for the secondary growthin crystal grains of the microcrystalline ZrO film, which is to bedielectric film 103.

According to the experiment by the inventors, a TiO film having a filmthickness of 1 nm or more is amorphous shortly after its formation, butpolycrystalline after a TiN film is formed at a film forming temperatureof 450° C. The TiO film was identified as behaving as a conductor,rather than a dielectric because it shows no change in a SiO₂ equivalentoxide thickness (EOT: A film thickness obtained by equivalentlyconverting a capacitance per the unit area of an electrode using thepermittivity 3.9 of SiO₂). Therefore, a polycrystalline titanium oxidefilm obtained by heat treatment to a TiO film having a thickness of 1 nmor more is not a dielectric, but functions as part of an upperelectrode. This is thought to be caused by the facts of (1) the Schottkybarrier is originally low in the combination of titanium nitride andtitanium oxide, and (2) titanium oxide behaves as a semiconductor by thepresence of impurities or oxygen deficiency.

When first protective film 110 made of a TiO film is formed after thedielectric film is heat-treated for a few minutes at 400° C., theleakage current is identified to deteriorate as that in Reference A inFIG. 2. The leakage current is also found to increase at 350° C.However, no change in leakage current was acknowledged with heattreatment at 300° C. In other words, as to the film formationtemperature (250° C.) in the ALD method for a dielectric film, thetemperature of 300° C., which is higher by 50° C., leads a lesssecondary growth in crystal grains and is found to have no problem in apractical sense. Based on further evaluation, there is no practicalproblem unless a temperature over the film forming temperature of theALD method for a dielectric film by 70° C. is set. Because thetemperature range (210° C. to 280° C.) in the ALD method for themicrocrystalline ZrO film is consistent with the temperature range 210°C. to 280° C.) in the ALD method for the first protective film, anytemperature within the temperature range would fall within thetemperature difference of 70° C. Of course, it is preferable not toperform unnecessary heat treatment between the microcrystalline ZrO filmformation and the first protective film formation. As such, the firstprotective film made of a TiO film should be formed before thepolycrystallization of the ZrO film progresses.

According to the experiment by the inventors, the desirable effect ofthe protective film for reducing the leakage current of the TiO filmformed on the dielectric made of a ZrO film is manifested from a filmthickness of 0.4 nm, and is preferable to have a film thickness of 1 nmor more. When a film thickness is 1 nm or more, an amorphous stateduring film formation changes into a polycrystalline state at theforming temperature of upper electrode 111, and the formation of anenergy band with crystallization and the formation of an energy levelwithin a band gap derived from, for example, oxygen deficiency make itpossible for the TiO film to function as a conductor, i.e., electrode,thereby showing no change in the EOT. The TiO film, which is to be thefirst protective film, is preferable to have a thickness of 5 nm orless. When the thickness is over 5 nm, damage, such as crack, may occurdue to the crystallization of the TiO film itself, and the damage maytransfer to the dielectric film. In this sense, a desirable filmthickness of the first protective film formed on a dielectric made of aZrO film falls within the range from 0.4 to 5.0 nm. A film thicknessbelow 0.4 nm may not obtain the effect of reducing leakage current. Morepreferably, the film thickness of the first protective film is within 1to 2 nm.

A dielectric film having a higher permittivity can be obtained in theprogress of densification together with the secondary growth of crystalgrains in the ZrO film, by means of heating at the time of the formationof upper electrode 111 after forming first protective film 110 made of aTiO film. The heat treatment may be performed separately from theformation of upper electrode 111. As described above, a TiO film havinga thickness of 1 nm or more is also converted to polycrystalline at thetime of the formation of upper electrode 111. As a means for promotingthis crystallization, heat treatment under a reducing atmosphere is alsoeffective. For example, using ammonia (NH₃) as the reducing atmospheremay cause the reduction-elimination of organic impurities contained inan amorphous TiO film, the introduction of oxygen deficiency within theTiO film (transition to low oxidation state, i.e., TiO_(x) where x is apositive real number less than 2), or the introduction of nitrogenimpurities, by performing heat treatment for 2 to 20 minutes at 380° C.to 460° C., and thus can promote crystallization. Therefore, it iseffective to preliminarily convert a TiO film, which is first protectivefilm 110, and a microcrystalline ZrO film, which is to be a dielectric103, into a polycrystalline state by heat treatment under ammonia orhydrogen atmosphere before forming a TiN film, which is to be upperelectrode 111. Because TiCl₄ and NH₃ are used as source gases to form aTiN film, which is to be upper electrode 111, a method of performingheat treatment as preliminary treatment under ammonia atmosphere shortlybefore forming a TiN film after installing a semiconductor substratewithin a TiN film forming apparatus. In this case, the process can besimplified because heat treatment under reducing atmosphere within a TiNfilm forming apparatus can be performed.

A portion of a TiO film, which is first protective film 110, may benitrided. For example, by changing the TiO film to an oxynitride such asa TiON film or the quality of a film doped with nitrogen atom asimpurities, the conductivity of the protective film increases.Performing heat treatment under ammonia atmosphere as described abovecan actively develop nitriding. The N density in the TiO film fallswithin the range from 1E19 [atoms/cc] to 1E21 [atoms/cc], and,preferably, is about 1E20 [atoms/cc]. However, the doping amount can beadjusted in accordance with a manufacturing environment, referring tothe C-V characteristic of a capacitor actually manufactured.

In other words, a TiO film as the first protective film can be evaluatedas having a function of prohibiting a dielectric film which compriseszirconium oxide as a primary constituent from being directly exposed tothe reducing atmosphere with a high temperature, such as NH₃, whenforming a TiN electrode.

A poly-crystallized TiO film having a thickness of 1 nm or morefunctions not only as a protective film for reducing leakage current ofa dielectric, but also as a portion of an upper electrode. A TiO filmhaving a film thickness of 0.4 nm or more but less than 1 nm remains inamorphous state, not being poly-crystallized at the heat treatmenttemperature for forming an upper electrode. Therefore, this TiO filmdoes not function for an upper electrode, but functions for a protectivefilm for reducing leakage current.

EXPERIMENT 4

A TiN film, as a protective film, is now evaluated.

Although a TiN film is used as an upper electrode, it is not formed by aCVD method, but by an ALD method, which allows a film formation at atemperature where a secondary growth of crystal grains of amicrocrystalline ZrO film is not accompanied, and where an increase inoxygen deficiency within an dielectric film or a diffusion ofimpurities, such as nitrogen, into the dielectric film may hardly occur.The structure of a capacitor is identical to the structure depicted inFIG. 5, but both of first protective film 110 and upper electrode 111are TiN films, and the TiN film constituting first protective film 110is integrated at the film forming stage. Therefore, the final structureis identical to the structure depicted in FIG. 1.

First, all films up to a microcrystalline ZrO film are formed as inExperiment 3, and then first protective film 110 made of a TiN film,rather than a TiO film, is formed by an ALD method. Hereinafter, a TiNfilm formed by the ALD method is referred to as “ALD-TiN film”.

An ALD-TiN film, which is first protective film 110, is formed by an ALDmethod at 250° C. with a Ti precursor of titanium tetrachloride (TiCl₄)and a reaction gas of ammonia (NH₃), and has a thickness of 1 nm.Ammonia was supplied in a plasma state. Specifically, the film formingprocess by an ALD method includes a fundamental sequence of (1)introducing the Ti precursor in a reaction chamber where a semiconductorsubstrate is installed and then adsorbing the Ti precursor on thesurface of a dielectric film, as an atomic layer level, (2)nitrogen-purging the remaining Ti precursor in a gas phase, (3)introducing the plasmic ammonia and decomposing the adsorbed Tiprecursor, and (4) nitrogen-purging the remaining ammonia in a gasphase. The film formation was developed by repeating the fundamentalsequence of the above four steps until the film thickness becomes 1 nm.A film formation by the ALD method is desirable in that it has a betterstep coverage and is easily applicable to a three-dimensional structurebecause the formation uses a surface adsorption reaction. Becauseplasmic-change of ammonia generates nitrogen radical having a highenergy level and thus improves reactivity, a nitriding reaction can bepromoted even at a low temperature of 250° C. Here, TiCl₄ was used as aTi precursor, but the Ti precursor is not limited to TiCl₄. TDMAT(tetrakis(dimethylamino) titanium: Ti[N(CH₃)₂]₄) orTDEAT(tetrakis(diethylamino) titanium: Ti[N(C₂H₅)₂]₄) can be used. TheseTi precursor gases can be used singly, or in combination of two or morethereof. The reaction gas is not only ammonia, but also nitrogen gas(N₂), N₂+NH₃, N₂+H₂, etc. Even when TDMAT or TDEAT is used as a Tiprecursor, a reaction gas is supplied in a plasma state. The filmforming temperature was set at 250° C., but is preferable if it iswithin 210° C. to 280° C. No reaction will take place below 210° C., anda decomposition reaction will occur in a gas phase if a temperature isabove 280° C. In both cases, an ALD film formation is prohibited.

Then, a CVD-TiN film, which is to be upper electrode 111, was formed.Similar in lower electrode 102, the CVD-TiN film for upper electrode 111was formed to have a thickness of 10 nm by a CVD method at 380° C. to600° C., preferably at 450° C., in consideration of its application to athree-dimensional structure. By heat treatment during the formation ofthe upper electrode, an ALD-TiN film, which is first protective film110, becomes the upper electrode formed integrally with the CVD-TiN filmby the crystal growth and the reduction-elimination of organic mattersremaining in the ALD-TiN film, and the microcrystalline ZrO film ofdielectric film 103 changes into a polycrystalline ZrO film.

FIG. 7 illustrates the leakage current characteristic of the capacitorin this experiment. The horizontal axis indicates a voltage applied toupper electrode 111, and the vertical axis shows a leakage current valueper unit area, which corresponds to an applied voltage. Thecharacteristic identified by Reference A is a leakage currentcharacteristic when a ZrO film as depicted as Reference A in FIG. 2 hasa thickness of 6 nm. Reference G illustrates a leakage currentcharacteristic when the capacitor includes first protective film 110made of an ALD-TiN film with a thickness of 1 nm. When a voltage of +1 Vis applied, the leakage current without first protective film 110(Reference A) is 2E-2(A/cm²), and the leakage current with firstprotective film 110 (Reference G) is 7E-8(A/cm²). As specified from thecomparison in FIG. 7, a capacitor with first protective film 110 made ofan ALD-TiN film having a thickness of 1 nm decreases the leakage currentby five exponents, compared to one without first protective film 110,and shows dramatic improvement.

The first protective film made of an ALD-TiN film is desirable to have athickness of 0.4 nm or more. The first protective film having athickness of less than 0.4 nm eliminates the effect of preventingdamages, such as crack, caused by a secondary growth of crystal grainsof a ZrO film. It is desirable that the first protective film made of anALD-TiN film has a thickness of 5 nm or less. If the first protectivefilm has a thickness of more than 5 nm, heat treatment during theformation of a CVD-TiN film as upper electrode 111 increases stress overthe entire TiN film constituting first protective film 110 and upperelectrode 111, and increases leakage current of dielectric film 103,thereby causing the loss of function as a protective film.

According to Experiments 3 and 4, the formation of a titanium compoundfilm as a first protective film prohibits damages caused by a secondarygrowth of crystal grains of a ZrO film during the formation of a CVD-TiNas an upper electrode. Therefore, leakage current characteristic isfound to improve.

Other than the TiO film and the TiN film, a TiON film, for example, maybe formed as a titanium compound film, which functions as a firstprotective film. Moreover, it can be a laminating structure of a TiOfilm and a TiN film.

EXPERIMENT 5

One embodiment of the present invention, in which a protective film isapplied to a dielectric film having a ZAZ structure, is now explained.

In this experiment, it is explained a case where a TiO film is formed asa protective film.

FIG. 8 is a schematic cross-sectional view illustrating the structure ofa capacitor according to the invention, which is different from thestructure of FIG. 3 in that first protective film 110 made of a TiO filmis formed between upper electrode 111 and dielectric film 107. Thecapacitor structure in this experiment is called a TZAZ structurebecause a polycrystalline TiO film, i.e., first protective film 110, iscombined with a ZAZ structure.

First, as in Experiment 2, all processes up to the formation of adielectric film having a ZAZ structure are performed. The dielectricfilm of a ZAZ structure includes, on lower electrode 201, a ZrO film of3 nm in thickness, which is to be first dielectric film 105, an AlO filmof 0.5 nm in thickness, which is to be second dielectric film 106, and aZrO film of 3 nm in thickness, which is to be third dielectric film 107.Although the present experiment forms first dielectric film 105 andthird dielectric film 107 with the same thickness of 3.0 nm, the samethickness is not required. For example, first dielectric film 105 mayhave a film thickness of 5.0 nm while the third dielectric film has afilm thickness of 1.0 nm. As such, both films can have an asymmetricalstructure. Although the present experiment includes a single-layer AlOfilm inserted between the ZrO films, two or more layers of AlO films canbe inserted therebetween. However, because a polycrystalline state maynot be achieved to provide a high permittivity if the thickness of theZrO film between the AlO films decreases, it is desirable to make thethickness of each ZrO film to be 1 nm or more. The composite ratio,which is expressed as Z/(Z+M) where Z is the number of zirconium atomand M is the number of aluminum atom in the entire dielectric film, ispreferably 0.8 or more.

Then, as in Experiment 3, first protective film 110 made of a TiO filmwith a thickness of 1 nm is formed by an ALD method. At the stage offilm formation, the TiO film is amorphous as described above. As in eachexperiment above, a CVD-TiN film which is to be upper electrode 111, isformed with a thickness of 10 nm by a CVD method at 450° C., and,likewise, is patterned in the shape of the upper electrode. At the stageof the film formation of upper electrode 111, the ZrO film of the thirdand first dielectric films continues to change from a microcrystallinestate into a polycrystalline state through a secondary growth of crystalgrains, where the grain boundary is acknowledged by a transmissionelectron microscope.

I-V characteristic of the capacitor formed as above was evaluated, andis illustrated in FIG. 9. In FIG. 9, Reference H shows the I-Vcharacteristic of the capacitor measured after upper electrode 111 hasbeen formed, a Reference I shows the I-V characteristic of the capacitormeasured after the upper electrode has been formed and then additionallyannealed with nitrogen at 450° C. for six hours, and Reference J showsthe I-V characteristic of the capacitor measured after the upperelectrode has been formed, annealed with nitrogen at 450° C. for sixhours and annealed with hydrogen at 450° C. for two hours.

With respect to Reference H, the applied voltage is −2.0 V or +2.1 V tomeet the index current density of 1E-7 (A/cm²). It is understood thatthe characteristic improves compared to Reference F depicted in FIG. 6.

Reference I shows a better leakage current characteristic in that theapplied voltage is −2.5 V or +2.4 V to meet the index current density of1E-7 (A/cm²). This is a better result than that of Reference D in FIG.4. Furthermore, Reference J, which is a result after the 2-hour hydrogenannealing, has an applied voltage between −2.3 V and +2.2 V to meet thecurrent density of 1E-7 (A/cm²), and shows a dramatic improvement inleakage current characteristic, compared to Reference E in FIG. 4.

As such, a TZAZ structure according to the invention improves theresistance against hydrogen-annealing, compared to a known ZAZstructure. Therefore, for DRAM, the characteristic of transistors canthus be compatible with that of capacitors, and, accordingly, highreliability and a high yield rate can be accomplished for a device.

Furthermore, it is understood from the result in Experiment 4, atitanium compound film, such as a TiN film, which can be formed by anALD method, can replace a TiO film as first protective film 110 toaccomplish the same result.

In the invention, it is desirable to maintain the process temperature as300° C. or less at least from the ZrO film formation for the thirddielectric film to the titanium compound film for the first protectivefilm. Alternatively, after forming a ZrO film for first dielectric film105 and forming an AlO film for second dielectric film 106, a ZrO filmcan be formed as third dielectric film 107 after a 10-minute heattreatment at 380° C. under oxygen atmosphere and a 10-minute heattreatment at 450° C. under nitrogen atmosphere.

Leakage current can be further reduced by the densification and change adielectric film into polycrystalline state by preliminarily promotingthe secondary growth of crystal grains on a ZrO film as first dielectricfilm 105. In this case, an AlO film of second dielectric film 106functions as a protective film for first dielectric film 105, andprohibits damages caused by the secondary growth of crystal grains ofthe ZrO film as first dielectric film 105. Even if a damage, such ascrack, is somewhat introduced, the formation of a ZrO film as the thirddielectric film may help the third dielectric film to fill the crack,thereby eliminating the damage. It is obvious that the processtemperatures for all dielectric film formations can be set at 300° C. orlower, and the first protective film can be then formed. All filmsformed by an ALD method may be formed in different film formingapparatuses, but are preferable to be subsequently formed in one filmforming apparatus.

A ZAZ structure for a dielectric film is designed to obtain a desiredEOT. Typically, the film thickness of all ZrO films, i.e., the first andthird dielectric films, are adjusted to be 5 to 7 nm, and the AlO filmfor the second dielectric film is designed to have a EOT of 0.6 nm orless with respect to the total film thickness of the first and thirddielectric films.

EXPERIMENT 6

In this experiment, in order to further improve leakage currentcharacteristic, it is explained a capacitor including the structure ofExperiment 5 and a TiO film for a second protective film. The secondprotective film is formed between a CVD-TiN film for lower electrode 102and a ZrO film for first dielectric film 105.

FIG. 10 is a schematic cross-sectional view illustrating a flatcapacitor according to this experiment. Unlike to the structure of FIG.8, FIG. 10 includes second protective film 108 made of a TiO filmbetween a ZrO film of first dielectric film 105 and a CVD-TiN film ofsecond electrode 102.

First, semiconductor substrate 101 was installed in an ALD film formingapparatus, and a TiO film, which is to be second protective film 108,was formed in the same manner for producing first protective film 110described in Experiment 5. The film was formed by an ALD method at 250°C. with a Ti precursor of TTIP (titanium tetraisopropoxide: Ti(OCHMe₂)₄)and a reaction gas of ozone so as to have a thickness of about 0.5 nm byfive cycles of the ALD fundamental sequence. With respect to this filmthickness, the TiO film formed by an ALD is in an amorphous state.

Then, as in Experiment 5, a dielectric film comprising a primaryconstituent of zirconium oxide, i.e., first dielectric film 105 made ofa ZrO film with a thickness of 3 nm, second dielectric film 106 made ofaluminum oxide with a thickness of 0.5 nm and third dielectric film 107made of a ZrO film with a thickness of 3 nm are formed by an ALD method;first protective film 110 made of a TiO film is formed to have 1 nm inthickness; and upper electrode 111 made of a TiN film is formed to have10 nm in thickness by a CVD method. Although the present experimentforms first dielectric film 105 and third dielectric film 107 with thesame thickness of 3.0 nm, the same thickness is not required. Forexample, first dielectric film 105 can have a film thickness of 5.0 nmwhile third dielectric film 107 has a film thickness of 1.0 nm. As such,both films may have an asymmetrical structure.

I-V characteristic of the capacitor formed as above was evaluated, andis illustrated in FIG. 11. In FIG. 11, Reference K shows the I-Vcharacteristic of the capacitor measured after the upper electrode hasbeen formed, Reference L shows the I-V characteristic of the capacitormeasured after the upper electrode has been formed and then additionallyannealed with nitrogen at 450° C. for six hours, and Reference M showsthe I-V characteristic of the capacitor measured after the upperelectrode has been formed, annealed with nitrogen at 450° C. for sixhours and annealed with hydrogen at 450° C. for two hours.

Compared to the result in FIG. 9, the leakage current here is reduced inthe region of low electric field (±2 V range). Absent second protectivefilm 108, the applied voltage of +1 V indicates 3E-8(A/cm²) forReference H, 3E-9(A/cm²) for Reference I, 1E-8(A/cm²) for Reference J.In contrast, with the second protective film 108 prepared, the appliedvoltage of +1 V indicates 2E-8(A/cm²) for Reference K, 6E-9(A/cm²) forReference L, 9E-9(A/cm²) for Reference M, each being further improved.

A TiO film as the second protective film contributes the improvement ofthe leakage current characteristic if it has a film thickness of 0.4 nmor more. When the film thickness is 1 nm or more, the TiO film changesfrom an amorphous state into a polycrystalline state by heat treatment.The TiO film as the second protective film has substantially noinfluence on an increase in the EOT of the entire capacitor after heattreatment during the upper electrode formation, and appears to functionas a conductor like the first protective film. Based on the fact that aTiO film as the second protective film has substantially no influence onthe effect of improving the leakage current characteristic when the filmhas 1 nm or 2 nm in thickness, the effect of improving the leakagecurrent characteristic is thought to be saturated. Therefore, a TiO filmfor the second protective film is desirable to have 0.4 to 2 nm inthickness, and, more preferably, 0.4 to 1 nm in thickness.

As described above, the same TiO film poly-crystallized with a filmthickness of 1 nm or more may play different roles according to thelocation where it has been formed. For example, a TiO film for firstprotective film 110 may prevent, during heat treatment, any directexposure to TiCl₄ or NH₃ (and by-produced HCl or H₂) environment, whichis used for forming a CVD-TiN film for upper electrode 111, while thesurface of a dielectric film having a major constituent of zirconiumoxide is exposed.

Meanwhile, a TiO film for second protective film 108 is inserted betweenthe dielectric film and the lower electrode so as not only to furtherprohibit leakage current but also to improve the adhesion between lowerelectrode 102 and dielectric film 105, thereby preventing a partialpeel-off between the lower electrode and the dielectric film during heattreatment.

A TiO film for the second protective film can also promote thecrystallization of zirconium oxide, and is found to help manufacturing adielectric film with a higher permittivity if any.

FIGS. 16 and 17 illustrate the outcome of an X-ray diffraction (XRD) ofa dielectric film having a primary constituent of zirconium oxide with6.6 nm in thickness when there is a TiO film for the second protectivefilm (Sample 2) and when there is no TiO film (Sample 1). FIG. 16 showsan XRD result shortly after film formation (as depo.) FIG. 17 depicts anXRD result after heat treatment (annealing) for six hours at 450° C.under nitrogen atmosphere. This annealing represents the heat loadgenerated during the growth of a CVD-TiN for an upper electrode in aprocess for manufacturing DRAM or during the film formation of apolycrystalline SiGe film doped with boron (B) by a CVD method, asexplained later.

FIG. 18 illustrates the structures of the samples used in FIGS. 16 and17. These samples have TiN film 401 with 10 nm in thickness, which is tobe lower electrode 102, formed on a silicone substrate (not shown).Additionally, Sample 2 (FIG. 18B), which has the second protective film,includes TiO film 404 formed by the aforementioned ALD method with a Tiprecursor of TiMCTA so that the film has a thickness of about 0.5 nmformed by five ALD cycles. Then, for the dielectric film having aprimary constituent of ZrO, film 402 is formed by the aforementioned ALDmethod so as to contain approximately 3 at % of AlO in the compositeratio expressed by Al/(Al+Zr). Then, TiO film 403 for the firstprotective film is formed by an ALD method with about 1.0 nm in filmthickness, which was formed by ten ALD cycles. As to Sample 1 (FIG.18A), which has no second protective film, TiN film 401 for the lowerelectrode is formed on a silicone substrate (not shown), and film 402for the dielectric film having a primary constitute of ZrO is formeddirectly on TiN film 401. TiO film 403 for the first protective film islikewise formed.

In FIG. 16, as to a film shortly after film formation without heattreatment, Sample 2 with the second protective film has a high peakintensity around 30.5° where zirconium oxide is crystallized. In FIG.17, Sample 2 having the secondary protective film also has high peakintensity and a good crystallizability. It should be noted that Sample 2having the second protective film has a small rate of change in peakintensity, in particular, before and after annealing, while its absolutevalue is large (See Table 1). This shows that Sample 2 having the secondprotective film has already been crystallized (i.e., a primary growth ofcrystal grains) sufficiently shortly after film formation, and that asecondary growth of crystal grains is prohibited during the upperelectrode formation.

TABLE 1 Change in XRD Peak Intensity Before Annealing After Annealing(FIG. 16) (FIG. 17) Rate of Change Sample 2 0.87 1.00 1.15 Sample 1 0.250.59 2.34

Table 1 shows the peak intensities of an XRD identified in FIGS. 16 and17, and the rates of change evaluated therefrom.

While, in Experiment 6, a TiO film for second protective film 108 isformed by an ALD method, the inventors here found that the same resultcan be achieved in the case that a titanium oxide film is obtained byoxidizing the surface of the lower electrode 102 made of titaniumnitride prior to the film formation of a dielectric film having aprimary constituent of zirconium oxide. The surface of the lowerelectrode may be oxidized by exposing the surface to an O₃ atmospherefor about 10 to 30 minutes at 250° C.

The inventors also found that the same effect can be achieved byproviding a titanium oxide film formed by an additional ALD method on atitanium oxide film obtained by oxidizing the surface of the lowerelectrode.

The film thickness of the second protective film formed by an ALD methodis an important issue which should not be overlooked. For example, aDRAM device in FIG. 13 has a lower electrode which is divided by unitsof cell and is insulated. However, the second protective film formed byan ALD method is formed over the entire surface. That is, the film isalso formed between adjacent lower electrodes. If a TiO film for thesecond protective film formed by an ALD method has a thickness of 1 nmor more, the film would function as a conductor and thus short-circuitthe lower electrodes to destroy the desired function as a device.Therefore, for a device with the structure as depicted in FIG. 13, thefilm thickness of the second protective film formed by an ALD methodshould be less than 1.0 nm, preferably 0.5 nm or less.

However, given the fact that a film thickness representing thesaturation of the second protective film's effect of improving theleakage current characteristic is 1 nm or more, an ALD method beingperformed on the entire surface may not prohibit leakage current betweenadjacent capacitors although the method may help reducing the leakagecurrent of the dielectric film.

To avoid this issue, it is possible to compensate the film thicknessrequired as the second protective film by oxidizing the lower electrodebefore the formation by an ALD as described above.

The second protective film formed as above does not short-circuitadjacent lower electrodes surely because the TiO film formed byoxidizing the lower electrode is formed only on the lower electrode.

Alternatively, the second protective film may also be formed only byoxidization as described above. However, oxygen may substantiallyincrease the resistance of the lower electrode because oxygen diffusesalong the grain boundary of the titanium nitride film constituting thelower electrode. When oxidization cannot be sufficiently done to avoidsuch an increase in resistance and the oxidization of the lowerelectrode alone is not enough to obtain a sufficient film thickness oftitanium oxide as the second protective film, the combination ofoxidization and an ALD method is recommended.

The formation of the second protective film (the formation byoxidization and by an ALD method), the formation of the dielectric film(the formation by an ALD method) and the formation of the firstprotective film (the formation by an ALD method) may be performedsubsequently in the same reaction chamber, thereby simplifying theprocess.

FIG. 12 shows the relation between the EOT and the leakage currentcharacteristic with respect to +1 V of a TZAZT structure (β) asdescribed in Experiment 6 and a TZAZ structure (α) as described inExperiment 5. For a reference, a single layer structure (γ) of a ZrOfilm as described in Experiment 1 is depicted together.

As shown in FIG. 12, the TZAZ structure (α) and the TZAZT structure (β)satisfy the feature that an EOT over 0.9 nm has a value of 1E-7(A/cm²)or less. Therefore, for DRAM of F70 nm class, an EOT of 1.2 nm or lesscan be sufficiently achieved.

(Exemplary Embodiment)

In this exemplary embodiment, a semiconductor memory device in which aTZAZ structure or a TZAZT structure is applied to a three-dimensionalcapacitor is explained in reference to FIGS. 13 to 15.

Referring to a schematic cross-sectional view of FIG. 13, the entirestructure of DRAM as a semiconductor memory device is generallydescribed.

n-Well 202 is formed on p-type silicone substrate 201, and first p-well203 is formed within n-well 202. Second p-well 204 is formed on theregion with the exclusion of n-well 202, and is separated from firstp-well 203 by element isolation area 205. First p-well 203 and secondp-well 204 conveniently represent, respectively, memory cell area MCwhere a plurality of memory cells is arranged and peripheral circuitarea PC.

First p-well 203 has switching transistors 206 and 207 including gateelectrodes which are to be word lines with components of each memorycell. Transistor 206 includes drain 208, source 209, and gate electrode211 with gate insulation film 210 inserted therebetween. Gate electrode211 has a polycide structure laminating tungsten silicide onpolycrystalline silicone or a polymetal structure laminating tungsten.

Transistor 207 includes common source 209, drain 212, and gate electrode211 with gate insulation film 210 inserted therebetween. The transistoris covered by first interlayer insulation film 213.

To be connected to source 209, a contact hole installed on a certainarea of first interlayer insulation film 213 is filled withpolycrystalline silicone 214. Metallic silicide 215 is provided on thesurface of polycrystalline silicone 214. Bit line 216 made of tungstennitride and tungsten is provided to be connected to metallic silicide215. Bit line 216 is covered by second interlayer insulation film 219.

For the connection to drains 208 and 212 of the transistors, contactholes are formed on a certain area of the first and second interlayerinsulation films 213 and 219, and each contact hole is filled withsilicone to provide silicone plug 220. Conductive plug 221 made of metalis provided on the top of silicone plug 220.

A capacitor is formed to be connected to conductive plug 221. Thirdinterlayer insulation film 222 a and fourth interlayer insulation film222 b, which are to form lower electrodes, are laminated on secondinterlayer insulation film 219. Fourth interlayer insulation film 222 bis reserved on the peripheral circuit area, and lower electrodes 223 areformed in a crown shape on the memory cell area. Then, fourth interlayerinsulation film 222 b on the memory cell area is eliminated. Thecapacitor is configured to have dielectric film 224 which covers theouter wall exposed by removing fourth interlayer insulation film 222 band the inner wall of lower electrode 223, and upper electrode 225 whichcovers the entire memory cell area. Support film 222 c is provided on aportion of the side of the top portion of lower electrode 223. Supportfilm 222 c is to connect some of a plurality of the adjacent lowerelectrodes, and thus to increase its mechanical strength and avoid thecollapse of the lower electrodes themselves. Because there is a spacebelow support film 222 c, dielectric film 224 and upper electrode 225are also provided on the surface of the lower electrodes exposed to thespace. FIG. 13 depicts two capacitors 301 and 302. Lower electrode 223is made of titanium nitride (TiN) formed by a CVD, which has anoutstanding step coverage. The capacitor is covered by fifth interlayerinsulation film 226. The material for the plugs is changeable dependingon the lower electrode of the capacitor; the material for the plugs isnot limited to silicone, but can be made of the same material as thelower electrode of the capacitor or of a different material. Thestructure of dielectric film 224 and upper electrode 225 is described indetail with a manufacturing process later.

A transistor, which constitutes a peripheral circuit, includes source209, drain 212, gate insulation film 210, and gate electrode 211 onsecond p-well 204. A contact hole which is installed in a certain areaof first interlayer insulation film 213 is filled with metallic silicide216 and tungsten plug 217 so that the hole is connected to drain 212.First wiring layer 218 which is made of tungsten nitride and tungsten isprovided to be connected to tungsten plug 217. A part of first wiringlayer 218 is connected metallic via plug 227 to second wiring layer 230made of aluminum or copper. Metallic via plug 227 is configured topenetrate second interlayer insulation film 219, third interlayerinsulation film 222 a, fourth interlayer insulation film 222 b and fifthinterlayer insulation film 226. Upper electrode 225 of the capacitorarranged in the memory cell area is withdrawn as wiring 228 from acertain area to the peripheral circuit area, and is connected to secondwiring layer 230 made of aluminum or copper by intervening metallic plug229 formed in a certain area of fifth interlayer insulation film 226.DRAM is developed by repeating the steps, as necessary, of forminginterlayer insulation films, forming contacts, and forming wiringlayers.

FIG. 14 is a schematic plane view of FIG. 13 taken along line X-X,excluding the dielectric film and the upper electrode. The line Y-Y inFIG. 14 corresponds to the line X-X in FIG. 13. Support film 222 c,which covers the entire outside of each lower electrode 223, includes aplurality of openings 231 over the entire memory cell area in a way ofextending over a plurality of the lower electrodes. Each lower electrode223 is so configured that part of its circumference is in contact withany one of openings 231. The support film with the exclusion of theopenings is continuously configured so that the lower electrodes areconnected to each other via the support film. The support film alsohelps avoiding the collapse of the lower electrodes themselves becausethe film may relatively extend the horizontal length with respect to theaspect ratio, i.e., the vertical/horizontal ratio. When cells areminiaturized with a high degree of integration, the aspect ratio, i.e.,the vertical/horizontal ratio, of the lower electrode of the capacitorincreases, and would thus cause the collapse of the lower electrodeduring its manufacturing without a means to support the lower electrode.FIG. 14 shows an example of opening 231 overlapping six lower electrodeswith a central focus on an area between capacitors 301 and 302.Therefore, in FIG. 13, the upper portions of capacitors 301 and 302 andof an area between the capacitors 301 and 302, which correspond to theareas in FIG. 14, are configured to have no support film.

As such, with the support film prepared, a better film forming methodwith a better coverage is required to form a dielectric film and anupper electrode on the surface of the lower electrode below the supportfilm.

A process for manufacturing a capacitor according to the invention isnow described with the exclusion of the other processes in a method ofmanufacturing DRAM as the semiconductor memory device described above.FIG. 15 is a cross-sectional view of a process for manufacturing onecapacitor depicted in FIG. 13. For clarity, a transistor or a firstinterlayer insulation film on semiconductor substrate 201 is omitted.

First, as shown in FIG. 15A, first interlayer insulation film 219 isformed on semiconductor substrate 201 made of monocrystal silicon. Then,a contact hole is formed on a predetermined location, and barrier metalfilm 221 a and metal film 221 b are formed on the entire surface. Then,barrier metal film 221 a and metal film 221 b, which have been formed onthe second interlayer insulation film is removed by a CMP method to formconductive plug 221. Then, third interlayer insulation film 222 a madeof a silicon nitride film, fourth interlayer insulation film 222 b madeof a silicon oxide film, and support film 222 c made of a siliconnitride film are formed on the entire surface.

Then, as shown in FIG. 15B, cylinder hole 232 is formed in support film222 c, fourth interlayer insulation film 222 b and third interlayerinsulation film 222 a by lithography and dry etching. The cylinder holehas a circular plane profile having a diameter of 60 nm. The closestdistance from the adjacent cylinder hole is 60 nm. As such, the bottomsurface of the cylinder hole is exposed to the top surface of conductiveplug 221.

Then, as shown in FIG. 15C, TiN film 223 a, which is the material of thelower electrode of the capacitor, is formed on the entire surfaceincluding the inner surface of cylinder hole 232. The TiN film can beformed by a CVD method with source gases of TiCl₄ and NH₃ at a formingtemperature between 380° C. to 650° C. The forming temperature is 450°C. and the film thickness is 10 nm in this embodiment. Alternatively,the TiN film can also be formed by an ALD method using the same sourcegases. The formation of TiN film 223 a defines new cylinder hole 232 a.

Then, as shown in FIG. 15D, protective film 234 such as a silicon oxidefilm is formed on the entire surface to load cylinder hole 232 a. Then,TiN film 223 a and the protective film 234 formed on the top surface ofthe support film 222 c are removed by a CMP or dry etching method toform lower electrode 223.

Then, opening 231 is formed in support film 222 c (see FIG. 15E). Asillustrated in the plane view of FIG. 14, the pattern of opening 231overlaps with a part of fourth interlayer insulation film 222 b, a partof lower electrode 223, and a part of protective film 234 remaining inthe inside of the lower electrode. Therefore, dry-etching for formingopening 231 removes a portion of the top of lower electrode 223 and theprotective film 234 as well as the support film 222 c formed on fourthinterlayer insulation film 222 b.

Then, as shown in FIG. 15F, fourth interlayer insulation film 222 bexposed in opening 231 is removed. For example, an etching process usinghydrofluoric acid solution (HF solution) does not substantially etchsupport film 222 c because support film 222 c is made of a siliconnitride film, but removes all of protective film 234 and fourthinterlayer insulation film 222 b formed with a silicon oxide film.

Besides the area right under opening 231, the silicon oxide film belowsupport film 222 c is also removed because the etching uses a solution.Accordingly, lower electrode 223 and support film 222 c supporting lowerelectrode 223 remains hollow, and lower electrode 223 exposes itssurface.

During this etching process, third interlayer insulation film 222 a madeof a silicon nitride film functions as an etching stopper, preventingsecond interlayer insulation film 219 from being etched.

Then, as shown in FIG. 15G, dielectric film 224 and first protectivefilm 225 a are formed. First protective film 225 a and dielectric film224 have a TZAZ structure described in Experiment 5 or a TZAZT structureinto which the second protective film as described in Experiment 6 isintroduce. These films can be formed by an ALD method. These TZAZ andTZAZT structures are optimized to achieve a desired feature for eachparameter. A film formed by an ALD method has an excellent stepcoverage, and thus dielectric film 224 and first protective film 225 aare formed on the entire surface of the lower electrode exposed as beinghollow. As defined above, ‘ZAZ’ in TZAZ or TZAZT indicates a dielectricfilm having zirconium oxide as a primary constituent and containingaluminum oxide within the dielectric film. ‘T’ is a protective filmhaving as a primary constituent a titanium compound, in particulartitanium oxide (or titanium nitride formed at a low temperature for theupper located ‘T’).

Then, as shown in FIG. 15H, a TiN film is formed as upper electrode 225b. Like the lower electrode, upper electrode 225 b is formed by a CVDmethod at 450° C. with source gases of TiCl₄ and NH₃. The film thicknessis 10 nm. Because a TiN film formed by a CVD method has very excellentstep coverage, the film can approach the hollow space so that it may beformed on the entire surface of first protective film 225 a.

Because dielectric film 224 is heat-treated under the protection of aTiO or TiN film, which is first protective film 225 a, although upperelectrode 225 b is formed at 450° C., damages such as oxygen deficiency,peeling-off, or crack on dielectric film 224 can be prohibited asexplained in the previous experiments, and the problem of an increase inleakage current can be avoided.

Then, as shown in FIG. 15I, a boron-doped silicon germanium film (B—SiGefilm) is formed as second upper electrode 225 c. At the stage of formingupper electrode 225 b in FIG. 15H, a hollow state partially remains toleave spaces all around. If tungsten which is to be a plate is formed bya PVD method under this circumstance, spaces would remain around thecapacitor even at the final stage of fabricating a semiconductor devicebecause the spaces may not be all filled due to the inferior stepcoverage of the PVD method. These remaining spaces worsens themechanical strength, and causes a change in the characteristic of thecapacitor due to the stress occurred during the packaging of thesubsequent processes. Therefore, the object of forming the B—SiGe filmis to stuff, and thus eliminate, the remaining spaces and to improveresistance to mechanical stress.

The B—SiGe film can be formed by a CVD method with source gases ofgermane (GeH₄), silane (SiH₄) and boron trichloride (BCl₃). The B—SiGefilm formed by this method has an excellent step coverage, which allowsstuffing the hollow spaces. The CVD method requires 420° C. to 500° C.at a forming temperature and performs heat treatment for six hours tothe capacitor when it is formed in a batch manner in consideration ofproductivity. The heat treatment in this process was represented by thenitrogen annealing at 450° C. for six hours in Experiments 5 and 6. Evenif a heat treatment at a maximum temperature of 500° C. is performedduring the process of forming a B—SiGe film for second upper electrode225 c, the capacitor according to the methods in Experiments 5 and 6 canhave low leakage current.

After forming the B—SiGe film as second upper electrode 225 c, atungsten film (W film) is formed as third upper electrode 225 d so as tobe used as a power supply plate covering the entire memory cell area.Because the W film is formed by a PVD method at 25° C. to 300° C., itimposes no heat-related influence such as an increase in the dielectricfilm's leakage current. After that, as shown in FIG. 13, a semiconductordevice including DRAM is fabricated by performing a process of formingfifth interlayer insulation film 226 and subsequent processes.

As described above, upper electrode 225 shown in FIG. 13 with the entirestructure includes a polycrystalline TiO film (or TiN film) as firstprotective film 225 a, a polycrystalline TiN film as upper electrode 225b, a B—SiGe film as second upper electrode, and a W film as third upperelectrode 225 d, as depicted in detail in FIG. 15I. When a TiN film isformed as first protective film 225 a, it is not distinguished, beingintegrated with a polycrystalline TiN film which is to be upperelectrode 225 b. The structure and manufacturing method for a DRAMaccording to this exemplary embodiment are to fabricate a most-advanced,super integrated DRAM. The B—SiGe forming process is not necessary if acapacitor which does not require support film 222 c for preventingcollapse is used even if it has a three-dimensional structure.

In the DRAM formed in accordance to the embodiment, as described inExperiments 5 and 6, the deterioration in leakage current characteristicis dramatically improved compared to a known ZAZ structure even uponhydrogen-annealing performed for reducing the interface state of atransistor. Therefore, the transistor characteristic can be compatiblewith the capacitor characteristic in DRAM, and a device with highreliability and high yield rate can thus be fabricated.

For example, hydrogen annealing can be performed at 400° C. to 450° C.for 30 minutes to 5 hours under reducing atmosphere containing hydrogengas. It is desirable to perform hydrogen annealing at least afterforming the upper electrode, and more preferably after forming, forexample, the wiring for the peripheral circuit area.

The prevent invention includes the following aspect of embodiment.

I. A semiconductor device comprising a capacitor, the capacitorcomprising:

-   -   a lower electrode comprising titanium nitride connected to a        semiconductor substrate,    -   a dielectric film comprising a layer of zirconium oxide as a        primary constituent, the layer of zirconium oxide being in        contact with said lower electrode and covering said lower        electrode, and    -   an upper electrode comprising a titanium nitride film, the        titanium nitride film being in contact with said dielectric film        and covering said dielectric film,    -   wherein a protective film comprising titanium oxide as a primary        constituent is inserted in either, or preferably both, of the        interface between said upper electrode and said dielectric film        and the interface between said lower electrode and said        dielectric film.

II. The semiconductor device according to item I, wherein saidprotective film comprising titanium oxide as a primary constituent andbeing inserted in the interface between said upper electrode and saiddielectric film is a titanium oxide film with a film thickness of 0.4 to5 nm.

III. The semiconductor device according to item II, wherein saidprotective film comprising titanium oxide as a primary constituent andbeing inserted in the interface between said upper electrode and saiddielectric film is a polycrystalline titanium oxide film with a filmthickness of 1 to 5 nm and has conductivity.

IV. The semiconductor device according to items I to III, wherein saidprotective film comprising titanium oxide as a primary constituent andbeing inserted in the interface between said lower electrode and saiddielectric film has a film thickness of not less than 0.4 nm and notmore than 2 nm.

V. The semiconductor device according to items I to IV, wherein saiddielectric film comprising zirconium oxide as a primary constituent hasa portion of the dielectric film containing aluminum oxide.

VI. The semiconductor device according to item V, wherein saiddielectric film comprising zirconium oxide as a primary constituent hasa composite ratio of 0.8 or more, the composite ratio being indicatedZ/(Z+M) where Z is the number of zirconium atoms and M is the number ofaluminum atoms.

VII. The semiconductor device according to items I to VI, wherein saiddielectric film comprising zirconium oxide as a primary constituent hasa film thickness of 5 to 8 nm.

VIII. The semiconductor device according to items I to VI, wherein saiddielectric film has a stacked structure of a first dielectric film madeof a polycrystalline zirconium oxide film, a second dielectric film madeof an amorphous aluminum oxide film on said first dielectric film, and athird dielectric film made of a polycrystalline zirconium oxide film onsaid second dielectric film, and

-   -   wherein said protective film comprising titanium oxide is formed        on said third dielectric film.

IX. The semiconductor device according to item VIII, wherein the filmthickness of said first and third dielectric films in total falls withinthe range from 5 nm to 7 nm.

X. The semiconductor device according to items I to IX, wherein SiO₂equivalent oxide thickness (EOT) of said dielectric film is 1.2 nm orless.

XI. The semiconductor device according to items I to X, wherein saidlower electrode has a three-dimensional structure.

XII. The semiconductor device according to item XI, further comprising asecond upper electrode made of a silicon germanium film containing boronon said titanium nitride film for the upper electrode.

XIII. The semiconductor device according to items I to XII, wherein saidcapacitor has the leakage current characteristic of a current density of1E-7(A/cm²) or less when a voltage within the range of ±2 V is applied,after being annealed at 400° C. to 450° C. under reducing atmospherecontaining hydrogen gas after the formation of the capacitor.

What is claimed is:
 1. A semiconductor device comprising a capacitor,the capacitor comprising: a lower electrode comprising titanium nitrideconnected to a semiconductor substrate, a dielectric film comprising alayer of zirconium oxide as a primary constituent, the dielectric filmbeing formed on said lower electrode, an upper electrode comprising atitanium nitride film, the upper electrode being formed on saiddielectric film, and a protective film comprising titanium oxide as aprimary constituent and being inserted in at least one of the interfacebetween said upper electrode and said dielectric film and the interfacebetween said lower electrode and said dielectric film.
 2. Thesemiconductor device according to claim 1, wherein said protective filmis inserted in either of the interface between said upper electrode andsaid dielectric film and the interface between said lower electrode andsaid dielectric film.
 3. The semiconductor device according to claim 1,wherein said protective film is inserted in both of the interfacebetween said upper electrode and said dielectric film and the interfacebetween said lower electrode and said dielectric film.
 4. Thesemiconductor device according to claim 1, wherein said protective filmbeing inserted in the interface between said upper electrode and saiddielectric film is a titanium oxide film with a film thickness of 0.4 to5 nm.
 5. The semiconductor device according to claim 4, wherein saidprotective film being inserted in the interface between said upperelectrode and said dielectric film is a polycrystalline titanium oxidefilm with a film thickness of 1 to 5 nm and has conductivity.
 6. Thesemiconductor device according to claim 1, wherein said protective filmbeing inserted in the interface between said lower electrode and saiddielectric film has a film thickness of not less than 0.4 nm and notmore than 2 nm.
 7. The semiconductor device according to claim 1,wherein said dielectric film has a portion of the dielectric filmcontaining aluminum oxide.
 8. The semiconductor device according toclaim 7, wherein said dielectric film comprising zirconium oxide as aprimary constituent has a composite ratio of 0.8 or more, the compositeratio being indicated Z/(Z+M) where Z is the number of zirconium atomsand M is the number of aluminum atoms.
 9. The semiconductor deviceaccording to claim 1, wherein said dielectric film comprising zirconiumoxide as a primary constituent has a film thickness of 5 to 8 nm. 10.The semiconductor device according to claim 1, wherein said dielectricfilm has a stacked structure of a first dielectric film made of apolycrystalline zirconium oxide film, a second dielectric film made ofan amorphous aluminum oxide film on said first dielectric film, and athird dielectric film made of a polycrystalline zirconium oxide film onsaid second dielectric film, and wherein said protective film comprisingtitanium oxide is formed on said third dielectric film.
 11. Thesemiconductor device according to claim 8, wherein the film thickness ofsaid first and third dielectric films in total falls within the rangefrom 5 nm to 7 nm.
 12. The semiconductor device according to claim 1,wherein SiO₂ equivalent oxide thickness (EOT) of said dielectric film is1.2 nm or less.
 13. The semiconductor device according to claim 1,wherein said lower electrode has a three-dimensional structure.
 14. Thesemiconductor device according to claim 13, further comprising a secondupper electrode made of a silicon germanium film containing boron onsaid titanium nitride film for the upper electrode.
 15. Thesemiconductor device according to claim 1, wherein said capacitor hasthe leakage current characteristic of a current density of 1E-7(A/cm²)or less when a voltage within the range of ±2 V is applied, after beingannealed at 400° C. to 450° C. under reducing atmosphere containinghydrogen gas after the formation of the capacitor.